Summary Previous research shows that response inhibition measured on Go/No-Go tasks is abnormal in all ages of ADHD persons, and that methylphenidate improves performance and alters neural function. When electrophysiological indices of response inhibition (i.e., the N2 ERP) are compared between No-Go success and failure trials, ADHD persons show differences related to automatic processes involved with response inhibition thought to involve prefrontal brain activity. This conclusion is supported by fMRI studies that localize No-Go hemodynamic response to inferior lateral prefrontal cortex and medial cortex (i.e., anterior cingulate and premotor cortex). Therefore, the N2 measures the strength of inhibitory neural processing, likely reflecting activity in brain structures known to be abnormal in ADHD with respect to both structure and hemodynamic function. However, electrophysiology has not yet been used to measure behavioral inhibition in ADHD adults, or to compare ADHD adults and youth.